The invention disclosed herein relates generally to the continuous casting of metals and, in particular, to an apparatus for and a method of uniformly cooling a solidifying metal bar, such as an aluminum or steel alloy, as it is cast by an improved wheel-and-band type continuous casting machine.
Although recent advances in the art of cooling continuously cast metal bar are a significant improvement over previous cooling methods first utilized in the 1960's, such methods still do not achieve complete uniformity of cooling during the entire solidification process. Thus the major problem of inverse segregation, especially on the top or band-side surface of a cast alloy bar, still occurs and adversely affects the subsequent working characteristics of the bar.
A desirable property of almost any metal alloy, preferably in its as-cast condition, is a uniform distribution within the cast product of the constituents and impurities normally found in the alloy. As used herein, what applicants believe to be the meaning of these terms is their standard meanings in the art, that is, "constituents" means ingredients or elements which make up a metallurgical system or a phase or combination of phases which occur in a characteristic configuration in an alloy microstructure, while "impurities" means elements or compounds whose presence in any material is generally undesired. Constituents, as used herein, then, would include the materials combined into a metallurgical system to produce the particular type of alloy being cast but would not include the impurities, or undesired elements or compounds present in the cast metal. For example, Aluminum Alloys such as 6201 (Al. Assoc. Designation) or ALDREY and ALMELEC (European designations) contain about 0.5% to 0.9% silicon, 0.5% iron, and 0.6% to 0.9% magnesium plus minor amounts of other elements such as copper, manganese, zinc, etc. In any case, segregation of the components in the cast alloy makes it less suitable for subsequent processing such as forging or rolling into rod and then drawing into wire. As used herein, the term "segregation" also has what applicants believe to be its normal meaning in the art, that is, segregation is a term used to describe the nonuniform distribution or concentration of constituents (or impurities) which arises during the solidification of a metal. For example, a concentration or accumulation of impurities in various positions within a metal is referred to in the art as segregation.
In normal segregation, for example in steel alloys, the constituents (solute) in the iron (solvent) rejected from the freezing liquid accumulate at the advancing solid/liquid interface so that the constituents of lowest melting point concentrate in the last portions to solidify, but in inverse segregation, for example in aluminum alloys, this is reversed, for the liquid with high solute concentration becomes trapped in between the dendrites, thereby causing a decrease in concentration of solutes from the ingot surface toward the center. Inverse segregation, then, is a concentration of constituents or impurities to a higher degree near the outer surfaces (as compared to the interior) of an ingot or casting.
Prior art methods of continuously casting alloys have often provided cast products having a relatively high degree of segregation of impurities and alloying materials within, or on the surface of, the cast bar. Because of the high level of constituents and impurities in many aluminum alloys, inverse segregation usually occurs. Such uneven distribution of impurities and/or constituents within the cast bar makes it desirable that the total amount of same within the alloy be reduced, thus also reducing the peak concentrations, so that subsequent processing of the cast bar does not result in unacceptable internal or surface characteristics in the product manufactured from the cast bar. A reduction in the total amount of impurities, however, usually requires expensive additional refining of the alloy prior to casting and is sometimes commercially unfeasible or impractical altogether while sometimes the addition of particular constituents (i.e. alloying elements) is desirable or necessary. It is apparent then, that the solidification process should be somehow controlled so as to eliminate segregation before it occurs. Howver this is especially difficult in a continuous casting process of the wheel & band type wherein the moving mold is subject to a complex and stationary cooling system fitted into the small space around the mold.